Add AnonArgFlag to FunTy

The big payload of this patch is:

  Add an AnonArgFlag to the FunTy constructor
  of Type, so that
    (FunTy VisArg   t1 t2) means (t1 -> t2)
    (FunTy InvisArg t1 t2) means (t1 => t2)

The big payoff is that we have a simple, local test to make
when decomposing a type, leading to many fewer calls to
isPredTy. To me the code seems a lot tidier, and probably
more efficient (isPredTy has to take the kind of the type).

See Note [Function types] in TyCoRep.

There are lots of consequences

* I made FunTy into a record, so that it'll be easier
  when we add a linearity field, something that is coming
  down the road.

* Lots of code gets touched in a routine way, simply because it
  pattern matches on FunTy.

* I wanted to make a pattern synonym for (FunTy2 arg res), which
  picks out just the argument and result type from the record. But
  alas the pattern-match overlap checker has a heart attack, and
  either reports false positives, or takes too long.  In the end
  I gave up on pattern synonyms.

  There's some commented-out code in TyCoRep that shows what I
  wanted to do.

* Much more clarity about predicate types, constraint types
  and (in particular) equality constraints in kinds.  See TyCoRep
  Note [Types for coercions, predicates, and evidence]
  and Note [Constraints in kinds].

  This made me realise that we need an AnonArgFlag on
  AnonTCB in a TyConBinder, something that was really plain
  wrong before. See TyCon Note [AnonTCB InivsArg]

* When building function types we must know whether we
  need VisArg (mkVisFunTy) or InvisArg (mkInvisFunTy).
  This turned out to be pretty easy in practice.

* Pretty-printing of types, esp in IfaceType, gets
  tidier, because we were already recording the (->)
  vs (=>) distinction in an ad-hoc way.  Death to
  IfaceFunTy.

* mkLamType needs to keep track of whether it is building
  (t1 -> t2) or (t1 => t2).  See Type
  Note [mkLamType: dictionary arguments]

Other minor stuff

* Some tidy-up in validity checking involving constraints;
  Trac #16263

1 files changed, 6 insertions, 6 deletions

diff --git a/compiler/typecheck/TcPatSyn.hs b/compiler/typecheck/TcPatSyn.hs
index 822697faf6..d46ade1028 100644
--- a/compiler/typecheck/TcPatSyn.hs
+++ b/compiler/typecheck/TcPatSyn.hs
@@ -750,16 +750,16 @@ tcPatSynMatcher (dL->L loc name) lpat
                | is_unlifted = ([nlHsVar voidPrimId], [voidPrimTy])
                | otherwise   = (args,                 arg_tys)
              cont_ty = mkInfSigmaTy ex_tvs prov_theta $
-                       mkFunTys cont_arg_tys res_ty
+                       mkVisFunTys cont_arg_tys res_ty
 
-             fail_ty  = mkFunTy voidPrimTy res_ty
+             fail_ty  = mkVisFunTy voidPrimTy res_ty
 
        ; matcher_name <- newImplicitBinder name mkMatcherOcc
        ; scrutinee    <- newSysLocalId (fsLit "scrut") pat_ty
        ; cont         <- newSysLocalId (fsLit "cont")  cont_ty
        ; fail         <- newSysLocalId (fsLit "fail")  fail_ty
 
-       ; let matcher_tau   = mkFunTys [pat_ty, cont_ty, fail_ty] res_ty
+       ; let matcher_tau   = mkVisFunTys [pat_ty, cont_ty, fail_ty] res_ty
              matcher_sigma = mkInfSigmaTy (rr_tv:res_tv:univ_tvs) req_theta matcher_tau
              matcher_id    = mkExportedVanillaId matcher_name matcher_sigma
                              -- See Note [Exported LocalIds] in Id
@@ -848,8 +848,8 @@ mkPatSynBuilderId dir (dL->L _ name)
              builder_sigma  = add_void need_dummy_arg $
                               mkForAllTys univ_bndrs $
                               mkForAllTys ex_bndrs $
-                              mkFunTys theta $
-                              mkFunTys arg_tys $
+                              mkPhiTy theta $
+                              mkVisFunTys arg_tys $
                               pat_ty
              builder_id     = mkExportedVanillaId builder_name builder_sigma
               -- See Note [Exported LocalIds] in Id
@@ -956,7 +956,7 @@ tcPatSynBuilderOcc ps
 
 add_void :: Bool -> Type -> Type
 add_void need_dummy_arg ty
-  | need_dummy_arg = mkFunTy voidPrimTy ty
+  | need_dummy_arg = mkVisFunTy voidPrimTy ty
   | otherwise      = ty
 
 tcPatToExpr :: Name -> [Located Name] -> LPat GhcRn